The present invention relates to a microwave flat plate or planar antenna, and a method of manufacturing the same. The invention particularly, but not exclusively, relates to a tri-plate antenna structure.
Flat plate microwave antennas are an alternative to the more traditional parabolic or dish style of antenna, and typically provide a more compact, lightweight and cheaper design. Flat plate antennas typically comprise an array of radiating elements disposed in a plane, and one or two ground planes located above and below the radiating element plane, and each having a number of apertures located to correspond with the radiating elements.
U.S. Pat. No. 4,486,758 (de Ronde) discloses such a planar antenna which utilises a solid dielectric material to position the radiating element plane between the ground planes. This arrangement has the disadvantage of using high loss solid dielectric which reduces the antenna efficiency, together with the additional weight which this material represents. An alternative approach is to use a point distribution positioning method to space the radiating elements from the ground planes as disclosed in U.S. Pat. No. 4,614,947 (Rammos). In this arrangement, the radiating elements are carried on a dielectric sheet, which is separated from the ground planes by discrete spacers formed by embossments in the metal plates forming the two ground planes. However this type of arrangement is complex and costly to manufacture.
A more recent advance in this field is to use continuous foam spacers between the radiating elements and the ground planes. The use of these foam spacers is advantageous in that it has a relatively low dielectric loss constant as well as a low weight, and also allows the manufacture of ground planes without embossed spacers or separate spacer components thereby considerably reducing the cost of production of these antennas making mass production of flat plate antennas feasible. While foam spaced flat plate antennas work well at frequencies in the range of 6-12 GHz, when scaling these constructions to operate at higher frequencies new difficulties are encountered. For example the thickness of the foam spacer has to be reduced from about 1 mm for a Ku band system (6-12 GHz) to 0.4mm for a 30 GHz system, with a corresponding decrease in tolerance. It is difficult to produce and work with foam sheets of this thickness, making foam spacers impractical in flat plate antennas operating at these frequencies. In addition the relative density of a thinner foam is likely to increase, due to smaller bubbles and an increased sidewall to bubble volume ratio, resulting in more dielectric losses. The proportion of open cells is also likely-to increase, leading to problems with water absorption.
With the increasing importance of broadband service provision using wireless access technologies, there is a need for an improved flat plate antenna design which operates at frequencies exceeding 12 GHz but is also relatively cheap to produce and lightweight and easy to work with.
It is an object of the present invention to provide an improved flat plate antenna.
It is a further object of the present invention to provide an improved method of manufacturing flat plate antennas.
In a first aspect the present invention provides an antenna structure comprising:
a first dielectric layer carrying a plurality of probes;
a second dielectric layer comprising a metallised surface having a plurality of apertures corresponding to said probes, each aperture and probe forming a radiating element; and;
a number of dielectric spacers formed with said second dielectric layer, each said spacer being located between a said aperture and a said probe to maintain said layers apart.
Preferably spacers are embossed in the second dielectric layer.
Preferably the antenna structure further comprises a third dielectric layer comprising a metallised surface having a plurality of apertures corresponding to said probes and the apertures of the second dielectric layer, the first dielectric layer being located between the second and third dielectric layers.
Preferably said metallised surface is on the side closest to the first dielectric layer.
In a second aspect the present invention provides an antenna tri-plate structure comprising:
a first dielectric layer carrying a plurality of radiating elements;
a second and third dielectric layer each comprising a metallised surface having a plurality of apertures corresponding to said radiating elements;
a number of dielectrics spacers formed with said second and third dielectric layers, each said spacer being located between a said aperture and a said radiating element to maintain said layers apart
In a third aspect the present invention provides a method of manufacturing an antenna structure comprising:
forming spacers on a dielectric layer;
forming a metallised surface on said layer, said surface having a plurality of apertures;
locating said layer on a second dielectric layer carrying a plurality of radiating elements corresponding to said apertures, wherein each said spacer is located between a said aperture and a said radiating element to maintain said layers apart.
Preferably said forming comprises embossing the first dielectric layer.